Question

A glass is half full of water, and sits on a scale. If you now put the tip of your finger in the water and keep it there, what happens to the reading on the scale?

Answer 1

If you put anything in water - a boat, a rubber duck or just your finger, it will feel an upthrust force pushing up on it. This is the force which makes boats float, and is equal to the weight of water that is displaced by the object.

Newton's third law requires that if you apply a force to something, you will feel an equal and opposite force (every action has an equal and opposite reaction). So if the water is applying an upwards, upthrust force to your finger, your finger must be applying a downward force to the water.

So your gives that extra push and consequently weight is more on the scale.

Another way of looking at it is that water level rises when you put finger in,so the water exerts higher pressure and hence force on scale is higher.